If your slab is free to stretch and or shrink nothing will happen. If your slab is attached to some rigid structure, like the wall of a lower storey, Still nothing needs to happe but you would need to prevent the concrete from heating up while setting. If you don’t and the concrete gets hot while still (semi-) liquid tension will build after setting as the slab cools down.
This tension will cause cracks at irregular intervals and some may get quite wide. If you had provide shrinkage bars the tension would still build up and cracks will also appear only much closer to eachother and much narrower individually.
Know that unless you take some very elaborate precautions such as prestressing or inserting coolant ducts all concrete will crack. In fact construction engineers will reckon with the strength of the cracked structure when designing a concrete structure. The rebar only helps us make the cracks acceptable and more predictable.